In many instances the encounter between host and microbial cells can be, through a longstanding evolutionary association, a balanced interaction whereby both cell types co-exist and inflict a minimal degree of harm on each other. Disease will only ensue when this balance is disrupted from the microbe's perspective, and/or as a result of, for example, "unintended" (in an evolutionary sense) consequences of immune or other host cell activity. This project proposes to study these intricate cellular interactions occurring between oral epithelial cells and important plaque microorganisms. In Specific Aim 1, we will assemble a baseline transcriptional profile of gingival epithelial cells in co-culture with Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis, two important periodontal pathogens. Human DNA microarrays will be used as a platform technology in addition to bioinformatics, statistical and ontology tools, to uncover highly significant genes and pathways of the host that are modulated upon microbial-host interaction. We will then, in the second Specific Aim, investigate at the molecular level the contribution of specific microbial genes to these complex interactions using a microbial mutant analysis approach. These studies will dissect selected host pathways related to bacterial adhesion, invasion and toxicity, and will confirm the transcriptional data at the protein and functional level. In the third Specific Aim, we will use mixed microbiota to compare the host's response to a complex microbial challenge. This will be done to assess if synergistic or antagonistic effects can be ascribed to combinations of challenging microorganisms, and investigate if there is a protective effect associated with the "good" microflora, represented here by Streptococcus gordonii. Phenotypic assays will be complemented with transcriptional profiles to address the effect of an increasingly complex flora on the host's response to microbial challenges. These studies are anticipated to help dissect the complex and dynamic interaction between the oral microflora and its host, which may lead, in the long run, to the development of novel, rational and practical therapeutic, prophylactic and diagnostic applications. Relevance to public health: This work proposes to study the interactions that occur in the oral cavity between microbes and the mucosal surface. Using state-of-the-art technology, we intend to characterize the entire set of host genes and pathways that are modulated in response to relevant microbial challenges. This study is expected to provide new avenues for therapies and diagnostics for periodontal diseases. This is particularly timely considering the recent association between the oral health and certain cardio-vascular diseases. [unreadable] [unreadable]